1. Field of the Invention
The invention relates to crosslinkable compositions based on silane-crosslinking prepolymers, to processes for preparation thereof and to the use thereof as adhesives and sealants.
2. Description of the Related Art
Polymer systems having reactive alkoxysilyl groups have long been known. On contact with water or the humidity of the air, these alkoxysilane-terminated polymers, even at room temperature, are capable of condensing with one another with elimination of the alkoxy groups. One of the most important uses of materials of this kind is the production of sealants and adhesives.
For instance, adhesives and sealants based on alkoxysilane-crosslinking polymers, in the cured state, exhibit not only good bonding properties on some substrates but also very good mechanical properties, since they can be both tear-resistant and highly elastic. Compared to conventional silicone sealants, silane-crosslinking systems additionally have the advantages of overcoatability and lower propensity to soiling.
In many applications, preference is given to one-pack systems which cure on contact with air humidity. The crucial advantage of one-pack systems is, in particular, the very easy applicability thereof, since no mixing of various adhesive components by the user is required.
A particular disadvantage of these systems according to the prior art is the low reactivity of the corresponding MS or SPUR polymers toward moisture, which necessitates aggressive catalysis. The corresponding mixtures therefore typically contain considerable amounts of tin catalysts, which are a matter of toxicological concern.
It is advantageous to use what are called α-silane-terminated prepolymers having reactive alkoxysilyl groups bonded by a methylene spacer to an adjacent urethane unit. This class of compounds is highly reactive and requires neither tin catalysts nor strong acids or bases to achieve high curing rates on contact with air. Commercially available α-silane-terminated prepolymers are GENIOSIL® STP-E10 or STP-E30 from Wacker-Chemie AG.
The standard silane-terminated polymers have a crosslinkable silane function on the majority of their chain ends and thus—if they are based on polymers having a linear backbone—contain two silane functions per molecule.
Especially for low-modulus systems, i.e. sealants having a 100% modulus <0.5 MPa, however, it is advantageous to use polymer systems which, in the cured state, lead to particularly low crosslinking densities. This can be achieved in a particularly simple manner by not exclusively using the abovementioned standard, usually difunctional prepolymers, but rather mixtures of these polymers with polymers having only one crosslinkable silane function per molecule. Systems of this kind are described, for example, in EP-A 1 710 270, EP-A 1 710 280 or else EP-A 2 076 568.
However, a disadvantage of most standard silane-terminated prepolymer formulations having a comparatively low crosslinking density in the cured state is the fact that they often have only an only moderately good resilience. In other words, these materials exhibit only very incomplete relaxation after prolonged extension. However, it is exactly this property that sealants generally need to have. For example, ISO 11600 requires a resilience exceeding 60% or even 70% for elastic sealants.
The problem of inadequate resilience relates especially to systems based on the abovementioned α-silane-terminated prepolymers that are particularly advantageous in terms of their superior curing properties. The connection between low crosslinking density and low resilience is obvious, since dense crosslinking makes it impossible for the crosslinked polymer chains, even in the stressed state, to move with respect to one another. Permanent deformation, i.e. poor resilience, is thus prevented by a higher crosslinking density. However, higher crosslinking densities inevitably also lead to higher-modulus materials.